1. Field of the Invention
The present invention relates generally to a lithographic projection apparatus and specifically to a lithographic projection apparatus including corrective optics to reduce ellipticity error.
2. Description of the Related Art
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus commonlyxe2x80x94referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the Y direction or xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Also herein, the invention is described using a reference system of orthogonal X, Y and Z directions. Further, unless the context otherwise requires, the term xe2x80x9cverticalxe2x80x9d used herein is intended to refer to the direction normal to the substrate or the plane comprising the pattern (as provided by the patterning structure) or parallel to the optical axis of an optical system, rather than implying any particular orientation of the apparatus. Similarly, the term xe2x80x9chorizontalxe2x80x9d refers to a direction parallel to the substrate surface or the surface of a pattern, as generated by the patterning structure, or perpendicular to the optical axis, and thus normal to the xe2x80x9cverticalxe2x80x9d direction. In particular, the horizontal direction corresponding with said scanning direction will be referred to as the Y direction.
Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
Correct imaging of a pattern generated by a patterning structure in a lithographic projection apparatus requires correct illumination of the patterning structure; in particular it is important that the intensity of illumination proximal the plane of the pattern, as generated by the patterning structure, or proximal planes conjugate to said plane of the pattern be uniform over the area of the exposure field. Also, it is generally required that the patterning structure can be illuminated with off-axis illumination in a variety of modes such as, for example, annular, quadrupole or dipole illumination, to improve resolution; for more information on the use of such illumination modes see, for example, EP 1 091 252, incorporated herein by reference. Said illumination modes are, for example, obtained by providing a corresponding preselected intensity distribution in a pupil of the illumination system. To meet above-mentioned requirements, the illumination system of a lithographic projection system is generally quite complex. A typical illumination system might include: shutters and attenuators for controlling the intensity of the beam output by the source, which might be a high pressure Hg lamp or an excimer laser; a beam shaping element such as, for example, a beam expander for use with an excimer laser radiation beam to lower the radiation beam divergence; a zoomable axicon pair and a zoom lens for setting the illumination mode and parameters (collectively referred to as a zoom-axicon); an integrator, such as a quartz rod, for making the intensity distribution of the beam more uniform; masking blades for defining the illumination area; and imaging optics for projecting an image of the exit of the integrator onto the patterning structure. For simplicity, the plane of the pattern generated by the patterning structure, and planes conjugate to this plane in the radiation system and the projection system may be referred to hereinafter as xe2x80x9cimagexe2x80x9d planes. The illumination system may also include elements intended to correct non-uniformities in the illumination beam at or near image planes. For instance the illumination system may include diffractive optical elements to improve the match of the projection beam cross-section proximal the entrance face of the integrator rod with the shape of said entrance face. A diffractive optical element typically consists of an array of microlenses, which may be Fresnel lenses or Fresnel zone plates. Improving said match alleviates the problem of field dependent lithographic errors occurring in the patterned layer. Said matching may hereinafter be referred to as xe2x80x9cfillingxe2x80x9d of the integrator entrance face. A diffractive optical element may also be positioned, for example, in front of a beam shaping element, such as a zoom-axicon, to transform the angular distribution of radiation provided by an excimer laser beam into a preselected angular distribution of radiation for generating a desired illumination mode. For more information on illumination systems such as mentioned here, see for example EP 0747772 and U.S. Pat. No. 5,675,401, incorporated herein by reference.
Also, the illumination system may, for instance, include a filter partially transmissive to radiation of the projection beam with a preselected spatial distribution of transmittance, immediately before the plane of the pattern, to reduce spatial intensity variations.
However, known illumination systems still suffer from various problems. In particular, various of the elements used, especially diffractive optical elements and quartz-rod integrators, can introduce an anomaly of intensity distribution in a plane perpendicular to the optical axis of the radiation system or the projection system. For instance, in a plane proximal a pupil of the radiation system or the projection system, either the projection beam cross-section may be elliptical rather than circular, or the projection beam intensity distribution within the projection beam cross-section may, for instance, be elliptically symmetric rather than circularly symmetric. Both types of errors are referred to as xe2x80x9cellipticity of the projection beamxe2x80x9d or simply as xe2x80x9cellipticity errorxe2x80x9d, and typically lead to specific lithographic errors in said patterned layers. In particular, a patterned feature occurring in directions parallel to both the X and Y direction may exhibit, in the presence of ellipticity of the projection beam, different sizes upon exposure and processing. Such a lithographic error is usually referred to as Hxe2x88x92V difference. Also, a diffractive optical element used to improve filling of the integrator is generally only optimum for one setting of the zoom-axicon. For other settings, the integrator entrance face may be under-filled (the projection beam cross-section is smaller than the integrator entrance face), leading to substantial field dependent Hxe2x88x92V difference. For said other settings, the integrator entrance face may also be over-filled, leading to energy wastage. Also, 157 nm excimer lasers and other excimer lasers tend to have large divergence differences in X and Y directions which cannot be completely resolved using beam expander lenses while keeping the shape of the beam within acceptable dimensions.
It is an object of the present invention to provide an improved illumination system for use in a lithographic projection apparatus, and particularly an illumination system in which the ellipticity of the projection beam can be controlled or reduced and the filling of the integrator entrance face can be controlled, to alleviate the problem of Hxe2x88x92V difference.
This and other objects are achieved according to the invention in a lithographic apparatus as specified in the opening paragraph, characterized by
correction means for correcting an intensity anomaly of said intensity distribution,
wherein the intensity anomaly comprises two elongated sections distributed along said X and Y direction, respectively, having substantially different intensities, and
wherein said correction means comprises an optical element rotatable around the optical axis of said projection apparatus.
It is recognized in this invention that an optical element causing ellipticity of the projection beam may be rotated around the optical axis of the system comprising said optical element to counteract any ellipticity caused by another element traversed by the projection beam.
Ellipticity of the projection beam may, for instance, be corrected by a diffractive optical element provided in the illumination system. An illumination system may of course include more than one diffractive optical element. Dependant on the source and extent of the ellipticity in the projection beam, the diffractive optical element may be rotated to an appropriate angle to make use of any inherent ellipticity or may be manufactured with a specific degree of ellipticity, e.g. by making each microlens in an array asymmetric. Preferably the diffractive optical element completely removes ellipticity or reduces it to an acceptable level. The invention can thus provide a lithographic projection apparatus in which the rotational position of a diffractive optical element is oriented to counteract an ellipticity error.
It is another aspect of the present invention to provide a radiation filter which is partially transmissive to the projection beam radiation, with a transmittance distribution which counteracts an ellipticity error caused by other optical elements traversed by the projection beam. Typically, such a radiation filter will have an inherent elliptically symmetric transmittance distribution. In particular, such a partially transmissive radiation filter arranged in series with a supplementary partially transmissive radiation filter, both rotatable around the optical axis of said projection apparatus, can be used as means to generate and adjust an amount of counteracting ellipticity, as needed to compensate ellipticity caused by said other optical elements.
The present invention also provides a lithographic projection apparatus comprising:
a radiation system for providing a projection beam of radiation, said radiation system comprising an illumination system for adjusting angular and spatial energy distributions of said projection beam and for providing a preselected intensity distribution of said projection beam in a pupil of said illumination system;
a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate and being movable along X and Y directions in an X, Y-coordinate system defined in said apparatus;
a projection system for projecting the patterned beam onto a target portion of the substrate, and for projecting said intensity distribution onto a pupil of the projection system,
wherein said illumination system comprises an integrator; characterized by:
a diffractive optical element located before said integrator and comprising an array of microlenses, said array of microlenses having a focal plane comprising the focal points of said microlenses, and a diffractive plate element, said diffractive plate element being situated proximal said focal plane of said array of microlenses and comprising a pair of substantially parallel plates proximal each other, said parallel plates having a refractive power varying sinusoidally at substantially the same pitch as said array of microlenses.
The diffractive plate element may be used to control filling of the integrator entrance face by adjusting the positions of said parallel plates proximal each other along at least one direction substantially perpendicular to the optical axis of the illumination system for different zoom-axicon settings, thereby avoiding under- and over-filling. The substantially transparent parallel plates proximal each other may be made of quartz and the sinusoidal refractive power can be provided by giving the opposed faces of the plates a sinusoidal profile in one or two orthogonal directions.
According to another aspect of the invention there is provided a method of setting up a lithographic projection apparatus comprising:
a radiation system for providing a projection beam of radiation, said radiation system comprising an illumination system for adjusting angular and spatial energy distributions of said projection beam and for providing a preselected intensity distribution of said projection beam in a pupil of said illumination system;
a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate and being movable along X and Y directions in an X, Y-coordinate system defined in said apparatus;
a projection system for projecting the patterned beam onto a target portion of the substrate, and for projecting said intensity distribution onto a pupil of the projection system,
the method comprising the steps of providing correction means for correcting an intensity anomaly of said intensity distribution, wherein the intensity anomaly comprises two elongated sections distributed along said X and Y direction, respectively, having substantially different intensities, and adjusting the rotational position of at least one optical element, rotatable around the optical axis of said projection apparatus and comprised in said correction means, such that said anomaly is substantially compensated.
According to a further aspect of the invention there is provided a device manufacturing method using a lithographic projection apparatus comprising the steps of:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using a radiation system;
using patterning structure to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material, characterized by the steps of:
providing correction means for correcting an intensity anomaly of an intensity distribution in a cross-section of the projection beam proximal a pupil selected from the group of pupils comprising a pupil of said radiation system and a pupil of said projection system, wherein the intensity anomaly comprises two elongated sections distributed along two mutually perpendicular directions, respectively having substantially different intensities, the rotational position of at least one optical element, rotatable around the optical axis of said projection apparatus and comprised in said correction means, being oriented such that said anomaly is substantially compensated.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).